Brought my 4K footage into Premier and sync'd two external sources of audio with timecode feature in Premier. When I used "new sequence from clip," after right clicking the newly sync'd sequence, everything matched up. However, after making a 1920x1080 sequence and a copy/paste of the 4K audio/video, my sync starts to lag. Both sequences are set to 23.976fps. Not sure why this is happening. The 4K sequence is perfect, but when copied to a 1920x1080 sequence the audio and video drift out of sync. Not using proxies by the way. Help, this is frustrating. Does anyone have a work around or can share their workflow for 4K audio syncing?

Yea, it's extremely strange and frustrating. Everything was shot on canon c300mkii's (internal 4K). I transcoded the .MXF files to ProRes422 in Media encoder. But it was all shot on 4096x2160 (4K). The timecode was off on some interviews so I had to manually adjust. Thinking it was a bug or something I used Pluraleyes, and the 4K sync was perfect. Then after a copy and paste into a 1920x1080, the sync was slipping. It happens infrequently. I tried nesting and that didn't work either. This is my first time working with 4K so I'm new to this workflow. Also first time syncing with timecode. So it's a little new as well.